(1) Field of the Invention
The present invention relates to an optical modulator provided outside of a light source in order to modulate the light from the light source, in particular to an optical modulator restricting a photorefractive phenomenon in the optical modulator.
(2) Related Art Statement
A dense wavelength division multiplexing (DWDM) technology and high speed communication technology have been developed for optical communication systems corresponding to an increase in the demand for high speed, large capacity data communication systems recently. Particularly, although the modulation frequency of an optical modulator is mostly 10 GHz, high speed modulation more than 40 GHz would also be required from now on.
As the optical modulator which corresponds to high speed modulation, the combination of CW (Continuous Wave) laser and the Mach-Zehnder(MZ) type external optical modulator (hereinafter described as LN optical modulator) using the material with an electro-optic effect, such as lithium niobate(LN), have been proposed and put to practical use.
Because LN optical modulator has small wavelength dependency, it is suitable for application in DWDM type optical modulator. Also, because there is no modulation bandwidth limit of dielectric loss, it enables extremely high speed modulation.
Like the optical modulator of 40 GHz, by increasing the light input power inputted into an LN optical modulator for the long distance transmission, degradation of an extinction ratio, increase of an optical loss and fluctuation of the bias point are induced. Especially when the light input power is more than 10 mW, such problems become evident. As a result of studies by the present inventors, they found out that the major factor is that the stray light generated from the input part which inputs laser light to an optical modulator and from an optical waveguide in the optical modulator, and the signal light which passes through the optical waveguide, in particular, interfere mutually, a photorefractive phenomenon is generated, and grating is written at the optical waveguide part by spatial overlap of stray beam and propagating beam.
Such grating written at the optical waveguide will cause degradation of the extinction ratio by reflecting the signal light that passes through the optical waveguide, in a direction opposite to the traveling direction, or by reflecting it outward from the optical waveguide.
The photorefractive phenomenon means the phenomenon that exposure to light varies the refractive index of an electro-optic material. In particular, due to the characteristic that a charge transfer is generated in the material by light, when optical distribution causes spatial intensity distribution of light, re-distribution of charge occurs corresponding to said intensity distribution of light, and this uneven distribution of charge varies an internal electric field topically. Because the internal electric field varies the refractive index of the material, refractive index distribution of the material that corresponds to the intensity distribution of light is formed resultantly.
Further, the photorefractive phenomenon has the characteristic that the refractive index changes little by little when being continuously exposed to light, and a light scattering gets stronger and stronger as time goes by. Therefore, in drive of an optical modulator for many hours, the deterioration of the optical modulator characteristics, especially degradation of the extinction ratio, increase of the optical loss, etc. becomes prominent.
The present invention intends to solve the above problems, to restrict the photorefractive phenomenon caused by a stray light in the optical modulator, and to provide the optical modulator which improves the characteristics relevant to the extinction ratio or optical loss of a signal light.
Particularly, the photorefractive phenomenon tends to occur for the optical modulator having a Mach-Zehnder type optical waveguide since there are many opportunities of interference with the stray light due to escaping light from a branching point of the branching optical waveguide and longer optical waveguide active part that allows phase modulation to work on the signal light passing through the optical waveguide. Further, for the optical modulator having so called dual electrode construction which drive controls several optical waveguide active parts by an independent modulating electrode separately, it is necessary to keep enough distance between modulating electrodes for avoiding cross talk between said modulating electrodes. This makes the length of the waveguide after the branching point of the branching optical waveguide longer, which increases the chances of interfering with the stray light and the photorefractive phenomenon tends to occur as a result.